1. Field of the Invention
The present invention relates to a wet-type image formation apparatus, and in particular to a wet-type image formation apparatus controlling image formation conditions based on the image density of a patch image.
2. Description of the Related Art
An image formation apparatus adopting a wet-type electrophotographic method (hereinafter also referred to as a wet-type image formation apparatus) can form high quality images, because it uses toner with a smaller diameter than that in a dry-type electrophotographic method. As disclosed in Japanese Lain-Open Patent Publication Nos. 2010-204468 and 2010-204469, an ordinary wet-type image formation apparatus includes a control unit for setting image formation conditions to an optimal state. By setting the image formation conditions to the optimal state, occurrence of image noise (such as rivulets, rear edge shift, and deterioration of dot reproduction) can be suppressed, and high quality images can be formed.
One of the means for suppressing occurrence of image noise is to set a charging amount for toner in a liquid developer conveyed to a development portion to a value that is as high as possible. The toner having a high charging amount is rarely influenced by the movement of a carrier liquid, and can form a toner image that is faithfully in line with the shape of an electrostatic latent image. On the other hand, when the toner charging amount is set to be higher than necessary, development characteristics have a too small gradient. In this case, the amount of toner used for development in a limited development potential difference is decreased, and development efficiency is reduced.
When the type of a recording medium (printing object) is changed or the like, the target range of a conveying amount of the liquid developer (toner) conveyed to the development portion by a developer carrier is also changed. When the toner conveying amount is changed to be increased, the toner charging amount is set low. With this setting, the development characteristics have a large gradient, which can suppress a decrease in the amount of toner used for development in a limited development potential difference, that is, a reduction in development efficiency.
When the toner conveying amount is changed to be decreased, the toner charging amount is set high. Even if the toner charging amount is not changed, a decrease in the amount of toner used for development, that is, a reduction in development efficiency can be suppressed. However, when the toner charging amount is not changed, there is room for further decrease in the gradient of an inclined portion of the development characteristics. To improve image quality, it is desirable to set the toner charging amount high.
Irrespective of whether or not the target range of the toner conveying amount is changed, it is desirable to set the toner charging amount as high as possible. The conveying amount of the toner in the liquid developer, the viscosity of the liquid developer, toner particle size distribution, and the like tend to vary depending on individual differences in manufacturing and a change in an ambient environment of the apparatus. These parameters influence a toner conveying amount which allows implementation of high quality image formation. Therefore, it is desirable to set a maximum value within a range in which high quality image formation can be implemented in an environment where the apparatus is placed, as the toner charging amount.